Haha, you weren't the only one doing this. I used to alter the fire engine to also have some absurd weight but and engine powerful enough to still move it. Their was also a time scaling option somewhere which would slow the game down and put it into a "matrix"-esque mode.

I used to jump into the fire engine, get on the wrong side of the road on a freeway and activate the slow motion before driving head on into the traffic. Their was something mesmerising about smashing your way through the cars and seeing them lift off into the air and fly into the buildings on either side of the freeway.

Eventually the police would show up but what could they do against a fire engine that was as dense as a sky scraper!